1. Field of the Invention
The invention relates to apparatus and methods associated with color measurement and analysis technology and, more particularly, apparatus and methods for measuring color characteristics of object samples comprising metallic and/or pearlescent particles.
2. Description of Related Art
It is well-known that the term "color" as applied to electromagnetic radiation represents in part the relative energy distribution of radiation within the visible spectrum. That is, light providing a stimulus to the human eye, and having a particular energy distribution, may be perceived as a substantially different color than light of another energy distribution. Concepts relating to the characteristics of color and light waves are the subject of numerous well-known texts, such as Principles of Color Technology, Meyer, Jr. and Saltzman (Wiley 1966) and The Measurement of Appearance, Hunter and Harold (Wiley 2nd Ed. 1987).
In recent years, the capability of maintaining the "quality of color" has been of significant importance in various industries, such as, for example, the fields of graphic arts, photography and color film processing. In addition, maintaining the quality of color is of significant importance in manufacturing industries producing objects having colored surfaces. For example, the automotive industry requires relatively high accuracy in measuring color characteristics of painted surfaces for purposes of maintaining color quality and repeatability for manufacture and repair of pigmented finishes. For purposes of performing sample testing and other activities in furtherance of maintaining color quality, it is necessary to first determine an appropriate means for "measuring" and "describing" color. A substantial amount of research has been performed during the past 50 years with respect to appropriate methods and standards for color measurement and description.
For purposes of describing color, and from a purely "physical" point of view, the production of color requires three things: a source of light; an object to be illuminated; and, a means for perceiving the color of the object. The means for perceiving the color can be the human eye and brain or, alternatively, electrical and electromechanical apparatus such as photosensitive detectors and associated auxiliary devices utilized for detecting light. In general, it is desirable to provide a means for measuring color so as to assess the manner in which an image will appear to a human observer, or the manner in which an image will perform in a photographic or other type of reproduction printing operation.
Although human perception and interpretation of color can be useful, reliance on such perception and interpretation can be highly subjective. That is, human nature may cause one person's perception of the color of a particular object to be substantially different from the perception of another. In addition, eye fatigue, age and other physiological factors can influence color perception. Further, visual human perception is often insufficient for color description. For example, certain object samples may be visually perceived under one light source as substantially "matching", and yet may actually have very different spectral characteristics and may be perceived as "non-matching" under another light source. In view of the foregoing, it is desirable to employ color measurement and description techniques which are objective in nature, and capable of differentiating among object samples having different color characteristics.
Various devices have been developed and-are widely utilized to measure and quantitatively describe color characteristics of object samples. Many of these devices provide measurements related to the spectral characteristics of the samples. Described simplistically, when light is directed onto an object sample to be measured for color, the object may absorb a portion of the light energy, while correspondingly passing through or reflecting (if the object is opaque) other portions of the light. The color characteristics of the object sample will depend in part on the spectral characteristics of the object. That is, the effect of an object on light can be described by its spectral transmittance or reflectance curves (for transparent or opaque materials, respectively). These spectral characteristic curves indicate the fraction of the source light at each wavelength transmitted by or reflected from the materials. Such curves are a means for describing the effect of an object on light in a manner similar to the use of a spectral energy distribution curve for describing the characteristics of a source of light. Instruments utilized for generating such spectral characteristics curves are typically referred to as spectrophotometers.
Although the present invention is disclosed with respect to use in a spectrophotometer, it is worthwhile, for purposes of background, to describe the use of other color measurement devices. In particular, for purposes of background description of typical types of components employed in many color measurement devices, concepts associated with a reflectance densitometer are set forth in the following paragraphs.
In accordance with conventional optical physics, its is known that the proportion of light incident to an object sample and absorbed by such a sample is independent of the light intensity. Accordingly, a quantitative indication of the spectral characteristics of an object sample can be defined as the "transmittance" or "reflectance" of the sample. That is, the transmittance of a substantially transparent object can be defined as the ratio of power transmitted over light power incident to the sample. Correspondingly, for an opaque object sample, the reflectance can be defined as the ratio of power reflected from the object over the incident light power.
For collimated light, these ratios can be expressed in terms of intensities, rather than power. Furthermore, because of the nature of transmittance/reflectance and the optical characteristics of the human eye, it is sometimes advantageous to express these ratios in logarithmic form. Accordingly, one parameter widely used in color technology fields for obtaining a quantitative measurement or "figure of merit" is typically characterized as optical "density." The optical density of an object sample is typically defined as follows: EQU Optical Density=D=-log.sub.10 T or -log.sub.10 R (Equation 1)
where T represents transmittance of a transparent object and R represents reflectance of an opaque object. In accordance with the foregoing, if an object sample absorbed 90% of the light incident upon the sample, and the object were opaque, the reflectance would ideally be 10%. The density of such a sample would then be characterized as unity. Correspondingly, if 99.9% of the light were absorbed, the reflectance would be 0.1% and the density would be 3. Similarly, the density of an "ideal" object reflecting 100% of the light incident upon the object would be 0.
To provide a relative measurement of color, it is possible to utilize the principles of optical density, without requiring measurement or knowledge of the absolute values of total incident light intensity or reflectance. That is, for example, it is possible to obtain relative color measurements among a series of object samples by utilizing a particular geometric configuration of light, object sample and reflectance or transmittance detector for each measurement, and standardizing the measurements in some desired manner.
In brief summary, optical density is a measurement of the modulation of light or other radiant flux by an object sample, such as a given area of a photographic print. Density measurements provide a means to assess the manner in which an image will appear to a human observer, or the way an image will perform in a film processing operation. Density measurements can be utilized to produce sensitometric curves to evaluate various printing and reproduction characteristics, as well as utilization to control various photographic operations, such as film processing.
For purposes of measuring optical densities, it is well-known to employ a device typically characterized as a "densitometer." These densitometers are often categorized as either "reflection" densitometers, employed for optical density measurements of opaque objects, or are otherwise characterized as "transmittance" densitometers. Transmittance densitometers are employed for determining spectral characteristics of various non-opaque materials.
FIG. 1 illustrates a simplified schematic representation of a known reflection densitometer configuration 100. A configuration of this type is described in detail in the commonly assigned U.S. Pat. No. 5,015,098 issued May 15, 1991 to Berg et al . Densitometer apparatus of the type shown in FIG. 1 are characterized as reflection densitometers, and utilized to provide color density measurements of opaque materials as previously described.
Referring specifically to FIG. 1, and to numerical references therein, the densitometer apparatus 100 includes a light source unit 102 having a source light 104. With respect to optical density measurements in various industrial fields relating to the optical characteristics of colored surfaces, various standards have been developed for densitometer light source illuminants. For example, densitometer light source standards have previously been described in terms of a tungsten lamp providing an influx from a lamp operating at a Planckian distribution of 3000.degree. K. Other suggested standards have been developed by the American National Standards Institute ("ANSI") and the International Organization for Standardization ("ISO"). These source light densitometer standards are typically defined in terms of the spectral energy distribution of the illuminant. The source light 104 preferably conforms to an appropriate standard and can, for example, comprise a filament bulb meeting a standard conventionally known in the industry as 2856K ANSI. Power for the source light 104 and other elements of the densitometer apparatus 100 can be provided by means of conventional rechargeable batteries or, alternatively, interconnection to AC utility power.
The source light 104 projects light through a collimating lens 106 which serves to focus the electromagnetic radiation from the source light 104 into a narrow collimated beam of light rays. Various types of conventional and well-known collimating lenses can be employed. The light rays transmitted through the collimating lens 106 project through an aperture 108. The dimensions of the aperture 108 will determine the size of the irradiated area of the object sample under test.
Various standards have been defined for preferable sizes of the irradiated area. Ideally, the aperture 108 is of a size such that the irradiance is uniform over the entire irradiated area. However, in any physically realizable densitometer arrangement, such uniform irradiance cannot be achieved. Current standards suggest that the size of the irradiated area should be such that irradiance measured at any point within the area is at least 90% of the maximum value. In addition, however, aperture size is typically limited to the size of the color bar or color patch area to be measured, and is also sized so as to reduce stray light.
The light rays emerging from the aperture 108 (illustrated as rays 110 in FIG. 1) are projected onto the irradiated area surface of an object sample 112 under test. The sample 112 may be any of numerous types of colored opaque materials. For example, in the printing industry, the sample 112 may be an ink-on-paper sample comprising a portion of a color bar at the edge of a color printing sheet. Further, with respect to the illustrative embodiment of a spectrophotometer apparatus employing the principles of the invention as described in subsequent paragraphs herein, the sample 112 may be a portion of a painted surface comprising metallic or pearlescent particles.
As the light rays 110 are projected onto the object sample 112, electromagnetic radiation shown as light rays 114 will be reflected from the sample 112. Standard detection configurations have been developed, whereby reflected light is detected at a specific angle relative to the illumination light rays 110 projected normal to the plane of the object sample 112. More specifically, standards have been developed for detection of reflected light rays at an angle of 45.degree. to the normal direction of the light rays 110. This angle of 45.degree. has become a standard for reflectance measurements, and is considered desirable in that this configuration will tend to maximize the density range of the measurements. In addition, however, the 45.degree. differential also represents somewhat of a relatively normal viewing configuration of a human observer (i.e. illumination at a 45.degree. angle from the viewer's line of sight).
On the other hand, however, and as described in subsequent paragraphs herein with respect to an illustrative embodiment of a spectrophotometer apparatus employing the principles of the invention set forth herein, the angle of detection of reflected light may vary away from the standard angle of 45.degree. relative to the normal direction of the light rays. Further, as also described in subsequent paragraphs herein, light may be detected at multiple angles relative to the impingement of light rays upon the object sample to be measured. Details associated with the concept of measurements at multiple angles will be set forth in subsequent paragraphs herein.
For purposes of providing light detection, a spectral filter apparatus 116 is provided. The filter apparatus 116 can include a series of filters 118, 120 and 122. The filters 118, 120 and 122 are employed for purposes of discriminating the cyan, magenta and yellow spectral responses, respectively. That is, each of the filters will tend to absorb light energy at frequencies outside of the bandwidth representative of the particular color hue of the filter. For example, the cyan filter 118 will tend to absorb all light rays, except for those within the spectral bandwidth corresponding to a red hue. By detecting reflected light rays only within a particular color hue bandwidth, and obtaining an optical density measurement with respect to the same, a "figure of merit" can be obtained with respect to the quality of the object sample coloring associated with that particular color hue.
It is apparent from the foregoing that the actual quantitative measurement of color density or color reflectance is dependent in substantial part on the spectral transmittance characteristics of the filters. Accordingly, various well-known standards have been developed with respect to spectral characteristics of densitometer filters. For example, one standard for densitometer filters is known as the ANSI status T color response. The spectral response characteristics of filters meeting this standard are relatively wide band (in the range of 50-60 nanometers (nms) bandwidth) for each of the cyan, magenta and yellow color hues. Other spectral response characteristic standards include, for example, what is known as G-response, which is somewhat similar to status T, but is somewhat more sensitive to respect to yellow hues. An E-response represents a European response standard.
Although the filters 118, 120 and 122 are illustrated in the embodiment shown in FIG. 1 as the cyan, magenta and yellow color shades, other color shades can clearly be employed. These particular shades are considered somewhat preferable in view of their relative permanence, and because they comprise the preferred shades for use in reflection densitometer calibration. However, it is apparent that different shades of red, blue and yellow, as well as entirely different colors, can be utilized with the densitometer apparatus 100.
The spectral filters 118, 120 and 122 may not only comprise various shades of color, but can also be one of a number of several specific types of spectral response filters. For example, the filters can comprise a series of conventional Wratten gelatin filters and infrared glass. However, various other types of filter arrangements can also be employed.
The spectral filters 118, 120 and 122 are preferably positioned at a 45.degree. angle relative to the normal direction from the plane of the object sample 112 under test. In the particular example shown in FIG. 1, each of these filters is maintained stationary and utilized to simultaneously receive light rays reflected from the object sample 112.
As further shown in FIG. 1, the portion of the reflected light rays 114 passing through the filters 118, 120 and 122 (shown as light rays 124, 126 and 128, respectively) impinge on receptor surfaces of photovoltaic sensor cells. The sensor cells are illustrated in FIG. 1 as sensors 132, 134 and 136 associated with the spectral filters 124, 126 and 128, respectively. The sensors 132, 134 and 136 can comprise conventional photoelectric elements adapted to detect light rays emanating through the corresponding spectral filters. The sensors are further adapted to generate electrical currents having magnitudes proportional to the intensities of the sensed light rays. As illustrated in FIG. 1, electrical current generated by the cyan sensor 132 in response to the detection of light rays projecting through the filter 118 is generated on line pair 138. Correspondingly, electrical current generated by the magenta sensor 134 is applied to the line pair 140, while the electrical current generated by the yellow sensor 136 is applied as output current on line pair 142. Photoelectric elements suitable for use as sensors 136, 138 and 140 are well-known in the art, and various types of commercially-available sensors can be employed.
The magnitude of the electrical current on each of the respective line pairs will be proportional to the intensity of the reflected light rays which are transmitted through the corresponding spectral filter. These light rays will have a spectral distribution corresponding in part to the product of the spectral reflectance curve of the object sample 112, and the spectral response curve of the corresponding filter. Accordingly, for a particular color shade represented by the spectral response curve of the filter, the magnitude of the electrical current represents a quantitative measurement of the proportional reflectance of the object sample 112 within the frequency spectrum of the color shade.
As further shown in FIG. 1, the sensor current output on each of the line pairs 138, 140 and 142 can be applied as an input signal to one of three conventional amplifiers 144, 146 and 148. The amplifier 144 is responsive to the current output of cyan sensor 132 on line pair 138, while amplifier 146 is responsive to the sensor current output from magenta sensor 134 on line pair 144. Correspondingly, the amplifier 148 is responsive to the sensor current output from yellow sensor 136 on line pair 142. Each of the amplifiers 144, 146 and 148 provides a means for converting low level output current from the respective sensors on the corresponding line pairs to voltage level signals on conductors 150, 152 and 154, respectively. The voltage levels of the signals on their respective conductors are of a magnitude suitable for subsequent analog-to-digital (A/D) conversion functions. Such amplifiers are well-known in the circuit design art, and are commercially available with an appropriate volts per ampere conversion ratio, bandwidth and output voltage range. The magnitudes of the output voltages on lines 150, 152 and 154 again represent the intensities of reflected light rays transmitted through the corresponding spectral filters.
Each of the voltage signal outputs from the amplifiers can be applied as an input signal to a conventional multiplexer 156. The multiplexer 156 operates so as to time multiplex the output signals from each of the amplifiers 144, 146 and 148 onto the conductive path 158. Timing for operation of the multiplexer 156 can be provided by means of clock signals from master clock 160 on conductive path 162. During an actual density measurement of an object sample, the densitometer 100 will utilize a segment of the resultant multiplexed signal which sequentially represents a voltage output signal from each of the amplifiers 144, 146 and 148.
The resultant multiplexed signal generated on the conductive path 158 is applied as an input signal to a conventional A/D converter 164. The A/D converter 164 comprises a means for converting the analog multiplexed signal on conductor 158 to a digital signal for purposes of subsequent processing by central processing unit (CPU) 166. The A/D converter 164 is preferably controlled by means of clock pulses applied on conductor 168 from the master clock 160. The clock pulses operate as "start" pulses for performance of the A/D conversion. The A/D converter 164 can be any suitable analog-to-digital circuit well-known in the art and can, for example, comprise 16 binary information bits, thereby providing a resolution of 65K levels per input signal.
The digital output signal from the A/D converter 164 can be applied as a parallel set of binary information bits on conductive paths 170 to the CPU 166. The CPU 166 can provide several functions associated with operation of the densitometer apparatus 100. In the embodiment described herein, the CPU 166 can be utilized to perform these functions by means of digital processing and computer programs. In addition, the CPU 166 can be under control of clock pulses generated from the master clock 160 on path 172. However, a number of the functional operations of CPU 166 could also be provided by means of discrete hardware components.
In part, the CPU 166 can be utilized to process information contained in the digital signals from the conductive paths 170. Certain of this processed information can be generated as output signals on conductive path 176 and applied as input signals to a conventional display circuit 178. The display circuit 178 provides a means for visual display of information to the user, and can be in form of any one of several well-known and commercially-available display units.
In addition to the CPU 166 receiving digital information signals from the conductive paths 170, information signals can also be manually input and applied to the CPU 166 by means of a manually-accessible keyboard circuit 180. The user can supply "adjustments" to color responses by means of entering information through the keyboard 180. Signals representative of the manual input from the keyboard 180 are applied as digital information signals to the CPU 166 by means of conductive path 182.
In general, the most commonly used instruments for "measuring" color now in commercial use are spectrophotometers, colorimeters and densitometers. While the three types of instrumentation are employed to measure reflected or transmitted light, a spectrophotometer typically measures light at a number of points on the visible spectrum, thereby resulting in a curve. With reference to FIG. 1, a spectrophotometer may have a similar configuration to the densitometer 100, but instead of having only three pairs of filters and photodiodes, a spectrophotometer may have, for example, sixteen or more pairs of filter and photodiode configurations. Each of the filters would be associated with a substantially separate portion of the visible light spectrum, for purposes of obtaining a curve representative of reflectance (for opaque objects) characteristics of various object samples. Typically, with a spectrophotometer, the output variable represented by the curve (as a function of wavelength) represents a percentage reflectance value. A spectrophotometer is considered essential in the color formulation of many products. Such products can vary from solid, opaque objects (such as ceramics and metals) to transparent liquids, such as varnishes and dye solutions.
A colorimeter, in contrast to a spectrophotometer, typically is utilized to measure light in a manner similar to the human eye, i.e. with utilization of red, green and blue (or similar colors) receptors. Colorimeters are utilized for many applications, including the measurement of printed colors on products such as packages, labels and other materials, where a product's appearance may be considered substantially critical for buyer acceptability. Such colorimeters will typically provide output in the form of tristimulus values or, alternatively, in the form of other values which tend to relate more specifically to appearance attributes of colors. For example, chromaticity coordinates are often utilized.
Spectrophotometer and colorimeter apparatus are commonly used for purpose of measuring color characteristics of painted surfaces. purposes manufacturing painted surfaces having pigmented finishes, it can be somewhat difficult to readily obtain a color "match" with a standard for the painted surface. In this regard, a spectrophotometer or colorimeter apparatus can be utilized for purposes of providing data to assist in adjusting formula pigment compositions to provide optimal matching of pigmented finishes with color standards. The manipulation of formula pigment compositions is typically referred to as "shading."
With conventional paints or other types of pigment compositions, a "color quality" measurement with sufficient tolerances can typically be obtained through the use of a single light source illuminating the object sample to be tested from a single illuminant. Correspondingly, with such paints and other pigment compositions, it is typically sufficient to utilize a single reflectance angle or "angle of detection" for the light rays reflected from the colored surface.
However, in many industries today, more complex paints and other pigment compositions are being employed. For example, in industries such as the automotive industry, paints are being utilized which employ light-reflecting particles or "flakes."These types of paints and other pigment compositions are typically referred to as "metallic" paints. The majority of automobiles currently being manufactured employ such metallic painted surfaces. With such automotive paints, a primer is first applied to the surface of the automobile. A base coat is then applied over the primer. Such base coat comprises a colored paint having metallic particles dispersed within the paint. The metallic particles are composed of materials such as aluminum, coated mica and the like. After application of the base coat comprised of the metallic particles, a "clear" coat is then applied. The clear coat essentially blocks ultraviolet light and maintains a gloss appearance for the painted surface.
Another type of color material which is currently being employed in the automotive industry comprises what is known as "pearlescent" paint. Pearlescent color materials typically utilize a three-coat system, in addition to the primer applied to the surface. A second coat is provided between the base coat and the clear coat. In part, the second coat can also include pearlescent particles.
With metallic color materials, a "lightness" or color change is exhibited with the viewing angle. That is, such color materials change visual appearance relative to the angle of view. With pearlescent materials, an actual color change is exhibited with a change in the viewing angle. Such metallic and pearlescent materials are particularly desirable with finishes for products such as automobiles, because the change in visual appearance tends to accentuate the contours of the automobile. That is, a change in the lightness or color will tend to cause the painted product to appear to have greater curvature. Accentuation of curvature, for aesthetic purposes, tends to increase the "glamour" of the product.
With metallic and pearlescent color materials, a single color measurement reading at a given angle for a spectrophotometer or colorimeter will not provide a color "quality" measurement which is sufficiently accurate to characterize the color characteristics of the paint. For purposes of fully characterizing the color characteristics of metallic or pearlescent materials, measurements at several different angles of view must typically be obtained. A substantial amount of developmental work has been undertaken with respect to determination of appropriate measuring techniques for determining color characteristics of metallic or pearlescent materials. For example, three-dimensional plots of spectral curves as a function of viewing angle have been obtained. However, such plots of spectral curves are substantially impractical for purposes of industrial color matching and control. Instead, development has been directed toward determining what could essentially be characterized as a "least" number of measurement angles providing sufficient information with respect to goniophotometric characteristics of a color, consistent with visual response, and sufficient to draw conclusions within industrial color matching tolerances.
For purposes of providing multiple viewing angles for metallic or pearlescent color materials, various types of procedures are known in the art. For example, it is known to utilize a detection arrangement comprising a fixed viewing angle relative to the direction of illumination, with rotation of the painted sample to be analyzed. Such arrangement is commonly known in the art as "object modulation." However, as readily apparent, such object modulation is substantially difficult with any object of relatively large size. Also, tolerances associated with the apparatus for rotation of the object sample under test must be extremely small, and large and expensive equipment is required for such rotation.
When the viewing angle is essentially "moved" to different positions, relative to the direction of illumination from a light source, such an arrangement is typically characterized as "detector modulation." Correspondingly, a third arrangement is commonly referred to as "source modulation." With source modulation, the object sample under test and the viewing angle are fixed, while the direction of illumination is varied. In any event, prior studies appear to have indicated that the measured color of such metallic or pearlescent color materials is primarily a function of the angle relative to the specular angle.
From the substantial prior development which has occurred with respect to characterization of goniophotometric characteristics of metallic and pearlescent color materials, it has essentially been concluded that for purposes of practicality, such color materials can be effectively characterized by measurement at three angles. This determination has been made in part through experimentation, as well as through various calculations. For example, within the prior art, the light reflected from a metallic finish has been characterized as being capable of analysis on the basis that such light has a diffused component (for multiple reflections with flake and other pigments) and a flake component (from a single reflection from a flake). The flake component, in addition to having an overall magnitude, can be characterized as having an angular distribution dominated by a single statistical characteristic of the flake orientation. With this single statistical characteristic, only two measurements are required for quantification of the same. When the metallic or pearlescent color materials are viewed from a direction substantially away from the specular angle, the color characteristic measurement is dominated by the diffused component. Correspondingly, the flake component dominates when viewing from a direction relatively close to the specular angle. Accordingly, it is argued that one angle of measurement should be substantially away from the specular angle, while another angle should be as close to specular as possible, while excluding "first surface" reflection. The third angle should be intermediate the other two angles at a location where the flake component is both significant and significantly different from that in the other measurements.
Other studies have involved the goniocolorimeteric characteristics of a substantial number of metallic colors covering several flake types and color pigmentations. Measurements were obtained of these metallic colors with respect to lightness, hue and chroma as a function of normalized angle. In these studies, the lightness response exhibited a relatively complex structure, and was thus chosen for polynomial modeling. Residual errors observed for each model were a measure of the information not obtainable from the "fit" of the model. It was found that linear models were relatively inadequate in modeling the curvature of the response, thereby leaving a relatively large residual error. Quadratic models having three appropriate chosen measurement angles were characterized as dramatically reducing the average residual sum of squares. Correspondingly, the use of four angles gave a relatively slight further reduction in a residual sum, although not sufficient so as to justify the increased complexity which is required in data handling from four measurement angles.
In summary, several previous studies have indicated that measurement of the optical properties of metallic or pearlescent color materials at only two specified angles can provide some useful characteristics. Such arrangements are discussed in Armstrong, Jr. et al, U.S. Pat. No. 3,690,771 issued Sep. 12, 1972. However, it has generally been accepted in the industry today that three angles of measurement are substantially optimal for purposes of determining color characteristics of metallic and pearlescent color materials. For example, Alman, U.S. Pat. No. 4,479,718 issued Oct. 30, 1984, is directed to an instrument for determining color characteristics of paints having reflecting flakes. The Alman patent describes an arrangement whereby the paint sample is illuminated by a single light source, with the light reflected by the paint sample being detected by a series of three detectors positioned at various angles. By utilizing the combined reflectance measurements of the three detectors, a "compensation" for the differing metallic reflecting qualities is achieved.
Alman further describes the concept that reflectance factors can be utilized to calculate color descriptor values for purpose of specifying color and color difference. Tristimulus values of a color can be calculated by combining the reflectance factor data with data on the sensitivity of the human eye and the irradiance of a light source, all as functions of wave length in the visible spectrum. The tristimulus values can be utilized to calculate color descriptors which relate to visual perception of color and color difference. For example, one set of descriptors which can be utilized are the CIE L*a*b* perceptual color scales as recommended by the International Commission on Illumination.
Transformations of the tristimulus values can be used to calculate various perceptual color values in accordance with equations set forth in the Alman patent. Alman further describes various theory associated with the determination of mathematical models for purposes of determining an optimal number of angles for purposes of measurement. Alman concludes that three properly selected measurement angles provide an optimized selection for purposes of determining maximum information on metallic color for relatively minimum measurement effort. Alman also describes the concept of providing an incident light source positioned at angle of 45.degree. relative to a surface comprising a paint film. Three detectors are position at three different angles, namely 15.degree., 45.degree. , and 110.degree. as measured from the specular angle. Although Alman describes the concept of employing a single light source with multiple detectors, apparatus associated with operation of the detectors is not specifically described within the application.
In another arrangement, Steenhoek, U.S. Pat. No. 4,917,495 issued Apr. 17, 1990, describes a colorimeter and method for characterizing optical properties of a colored surface comprising metallic or pearlescent particles. The Steenhoek arrangement utilizes three multi-angular spectrophotometric measurements to derive color constants for the surface.
The Steenhoek colorimeter specifically utilizes three sources of illumination comprising three separate lamps. The output of the lamps is collimated by achromatic source lenses mounted at the corresponding focal lengths from the lamp filament. Collection optics are included which comprise achromatic collection lenses mounted at twice their corresponding focal lengths from a sample surface. A monochrometer, comprising a diffraction grating and a silicone diode array detector, is mounted opposite to the sample side of the lens. An entrance slit to the monochrometer is mounted at a distance of one focal length from the lens. Such an arrangement permits only light which is very nearly collimated to pass through the entrance slit, permitting only light scatter at or about 45.degree. from the sample normal to enter the monochrometer.
After passing through the entrance slit, light diverges until it impinges upon the diffraction grating. At the diffraction grating, the light is dispersed and refocused onto a silicone diode array detector having 12 detecting elements. Each of the detecting elements includes an associated amplifier which converts diode current to a voltage signal. The 12 signals are then multiplexed and digitized by an analog to digital converter. All of the functions of the colorimeter are controlled by a microcomputer, and measurement data derived from the instrument is displayed on an LCD display. Within the colorimeter, the sample to be measured is sequentially illuminated from minus 30.degree., 0.degree. and 65.degree. as measured from a sample normal. Light reflected from the sample is detected at 45.degree. as measured from the sample normal.
In accordance with the foregoing, concepts associated with multi-angular measurements of color materials employing metallic or pearlescent particles are relatively well known. However, these known systems typically require a substantial number of optical elements. For example, although not specifically described in the Alman patent, it appears that each of the detector arrangements shown therein would require separate detection circuitry for providing electrical signals representative of the detected light rays. Correspondingly, with respect to the Steenhoek patent, three illumination sources are required. Still further, although Steenhoek is described as a portable device, it is not clear that the device described in the Steenhoek patent would include a power source, except for a source separate from the primary unit.